1. Field of the Invention
This invention relates to means and method for detecting impurity concentration in a flowing gas stream.
2. Description of the Related Art
The rapid expansion of vapor-phase processing techniques, e.g., chemical vapor deposition, in the semiconductor industry has been associated with the deployment and use of manufacturing equipment which is totally reliant on the delivery of ultra-high purity process gases at the point of use in the semiconductor manufacturing facility. Currently, over 5 billion dollars worth of such equipment is in use.
Despite the widespread commercial employment of such vapor-phase processing equipment, little effort has been focused to date on the development of systems for monitoring purity of gas streams in the process system.
As a result of the absence of commercially suitable gas impurity monitoring systems, there is a recurrence of circumstances where a large number of wafers have been processed in the vapor-phase deposition reactor before it is recognized that compositional changes in the process gas stream flowed to the reactor are leading to high rates of rejection. Such high rates of rejection in turn significantly lower the efficiency and productivity of the semiconductor manufacturing plant, and generate substantial losses of potential product. The resulting off-spec microcircuitry articles thus constitute scrap which must be reworked, if this is even feasible, or else discarded as waste.
Accordingly, there is a pressing need in the semiconductor manufacturing industry to provide commercially viable systems for continuously measuring gas purity at the point of use. Such purity measurements can be used to alter process conditions that would otherwise lead to production problems, e.g., by diverting the impurity-containing gas stream to suitable treatment prior to its ultimate use in the deposition process.
In the context of general industrial processes, such as petroleum refining, wastewater treatment, biopharmaceutical production, etc., a variety of impurity monitoring and detection systems have been developed to detect fluid phase impurities, using sampling of a side stream, or slip stream, of the main flow stream for impurity concentration determination. The sampled side stream typically is flowed through the monitoring and detection apparatus and then discarded. In the field of semiconductor manufacture, such wastage is highly detrimental to the economics of the semiconductor production process when the gas stream, as is generally the case, contains costly reagent materials, e.g., organometallic source reagents for metal deposition on a substrate. Further, many gas streams employed in semiconductor manufacturing are highly hazardous in character, so that their waste presents significant difficulties in handling, treatment, and disposal.
Considering the impurities which are present in gas streams involved in semiconductor manufacturing, it is to be noted that the growth of high quality thin film electronic and opto-electronic cells by chemical vapor deposition or other vapor-based techniques is inhibited by a variety of low-level process impurities. These impurities affect both product semiconductor defects and yield.
Specifically, at least two types of contamination are significant, viz., particulate contamination and chemical contamination. Particulate contamination has been successfully addressed by a variety of filtration and collection methods and apparatus (see Malczewski, M. L., et al, "Measurement of Particulates in Filtered Process Gas Streams," Solid State Technology, 28, 151-157, April 1986). Chemical contamination has not received similar attention. As mentioned, the monitoring devices which have been developed in other industries are ill-suited for application to semiconductor manufacturing operations.
In the semiconductor manufacturing operation, chemical impurities in reactive process gases can originate in the production of the source gas itself, as well as in its subsequent packaging, shipment, storage, and handling. Although source gas manufacturers typically provide analyses of source gas materials delivered to the semiconductor manufacturing facility, the purity of such gases may change. Such change may be due to leakage into or outgassing of the containers, e.g., gas cylinders, employed to package such gases. Alternatively, impurity contamination may result from improper gas container changes, leaks into downstream processing equipment, or outgassing of such downstream equipment.
Accordingly, the only comprehensive solution for consistent delivery of high purity gases for vapor processing operations in semiconductor manufacture is the development of commercially useful impurity detection systems for real-time measurement of critical impurity concentrations in semiconductor manufacturing process streams and the deployment of reliable point-of-use purification systems for purifying gas streams which are determined to contain impurity species in excess of allowable concentrations.
The presence of even small concentrations of impurity species in the process gas streams employed in semiconductor manufacturing is potentially deleterious. Even small levels of impurities on the order of parts-per-million (ppm) can cause inconsistent electrical properties in semiconductor devices manufactured by deposition techniques using impurity-containing gas streams.
It therefore is an object of the present invention to provide a system for detection of impurity concentrations in a flowing gas stream, which can be usefully employed in semiconductor manufacturing operations.
It is another object of the present invention to provide a detection system of such type, which is capable of providing real-time monitoring of process gas streams, so that immediate correction can be undertaken when impurity concentration levels exceed predetermined set point limits.
It is a further object of the invention to provide a system for detecting impurity species in flowing gas streams, which is employed in semiconductor manufacturing operations, and which does not require any side stream or slip stream sampling for its utilization.
It is a still further object of the invention to provide a system for detecting impurity concentrations in a flowing gas stream, which is readily calibrated and has a substantial continuous service life, e.g., on the order of at least six months.
Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.